Modeled and observed impacts of the 1997–1998 El Niño on nitrate and new production in the equatorial Pacific

Abstract

Abstract. The impact of the strong 1997-1998 E1Nifio event on nitrate distribution and new production in the equatorial Pacific is investigated, using a combination of satellite and in situ observations, and an ocean circulation-biogeochemical model. The general circulation model is forced with realistic wind stresses deduced from ERS-1 and ERS-2 scatterometers over the 1993-1998 period. Its outputs are used to drive a biogeochemical model where biology is parameterized as a nitrate sink. We first show that the models capture the essential circulation and biogeochemical equatorial features along with their temporal evolution during the 1997-1998 event, although the modeled variability seems underestimated. In particular, the model fails to reproduce unusual bloom conditions. This is attributed to the simplicity of the biological model. An analysis of the physical mechanisms responsible for the dramatic decrease of the biological equatorial production during E1 Nifio is then proposed. During the growth phase (November 1996 through June 1997), nitrate-poor waters of the western Pacific are advected eastward, and the vertical supply of nitrate is reduced due to nitracline deepening. These processes result in the invasion of the equatorial Pacific by nitrate-poor waters during the mature phase (November 1997 through January 1998). At that time, the central Pacific is nitrate limited and experiences warm pool oligotrophic conditions. As a result, the modeled new production over the equatorial Pacific drops by 40% compared to the mean 1993-1996 values. Then, while E1Nifio conditions are still present at the surface, the nitracline shallows over most of the basin in early 1998. Therefore the strengthening of the trade winds in May 1998 efficiently switches on the nitrate vertical supply over a large part of the equatorial Pacific, leading to a rapid return of high biological production conditions. Strong La Nifia conditions then develop, resulting in a biologically rich tongue extending as far west as 160øE for several months.

Seasonal and interannual variability of ocean color and composition of phytoplankton communities in the North Atlantic, equatorial Pacific and South Pacific

“…At the end of this warm episode (early 1998), the entire equatorial tongue of water had a low chlorophyll concentration. When upwelling started again, the biological response was immediate and intense, in the form of a sharp chlorophyll maximum right at the equator (Murtugudde et al, 1999;Radenac et al, 2001). Certainly, the zooplankton populations had been submitted to food shortage and high mortality during the El Niño episode, and when upwelling was restored, grazing pressure was low (tropical copepods have a generation time of about three weeks) and could not restrain the exponential growth of the phytoplankton.…”

Displacements and transformations of nitrate-rich and nitrate-poor water masses in the tropical Pacific during the 1997 El Niño

“…During the peak of the event, from November 1997 to January 1998 (Fig. 1c), more than 6 months after the El Nin˜o event started, the chlorophyll-rich region was reduced to its narrowest zonal extent (Radenac et al 2001) and high chlorophyll waters stretch from the Central American coast to the equatorial western basin (Murtugudde et al 1999). Unfortunately, there was a three-month gap (July 1997-September 1997 in the time series of ocean color data that corresponded to an essential transition phase of the onset of El Nin˜o.…”

“…Before the onset of the event, weak La Nin˜a conditions prevailed in the tropical basin in late 1996 (Fig. 1a) and chlorophyll rich waters (Muramaki et al 2000;Radenac et al 2001;Ryan et al 2002) extended across most of the equatorial basin. In May 1997, the satellite chlorophyll ( Fig.…”

“…Most previous studies focused on the equatorial zone. Observations and simulations have shown that the variability of nitrate, chlorophyll or new production is mainly controlled by the fast equatorial dynamics (Chavez et al 1998;Stoens et al 1999;Friedrichs and Hofmann 2001; Radenac et al 2001) and their analyses stressed the abrupt decrease of primary production. They did not closely examine the off-equatorial situation for which we hypothesize here that the low nitrate uptake rate characteristic of the HNLC ecosystem should delay the setting up of oligotrophic conditions that characterize El Nin˜o.…”

“…They did not closely examine the off-equatorial situation for which we hypothesize here that the low nitrate uptake rate characteristic of the HNLC ecosystem should delay the setting up of oligotrophic conditions that characterize El Nin˜o. In this study, a Lagrangian analysis (Blanke and Raynaud 1997) was applied to the outputs of a physicalbiogeochemical model (Radenac et al 2001) to address the following two issues. First, the goal was to describe the redistribution of surface water masses in the tropical basin during the 1997 El Nin˜o and secondly, to investigate the role of the slow nitrate uptake in the HNLC region during the growth phase of the event.…”